The present invention relates to the field of digital timekeeping systems with provisions for handling vital information during temporary power dropouts.
Recently, digital timekeeping or clock systems have been developed which count cycles of a reference frequency, e.g., 60 Hz, signal to generate binary signals indicative of time. These timekeeping systems have been employed in combination with such instruments as radio and television receivers and more recently with video tape recording systems to not only display the time but to automatically control the instrument at preset times. Because these timekeeping systems typically derive their power from the same source of power, i.e., the AC line input, as the instrument in which they are included, these timekeeping systems are susceptible to the loss of information during power dropouts.
While some timekeeping systems merely provide a particular type of display after a loss of line power to indicate that the timekeeping function has been interrupted, other systems employ a standby source of power, such as a battery, in conjunction with an auxiliary reference frequency oscillator to continue the timekeeping function during a loss of line power. While the use of a battery as a standby power source may be suitable in some applications, it has many disadvantages in others. Not only are batteries relatively expensive, but their replacement may be an annoying inconvenience to a user. Moreover, in some instruments, like a television receiver, where there may be high voltages stored even during the absence of line power, it may be dangerous for a user to replace an internally located battery without adequate safety provisions which may be relatively expensive. While provisions, like battery chargers, may make the replacement of batteries unnecessary, such provisions also undesirably add cost to an instrument.
In digital equipment, a variety of arrangements are known for storing information during the absence of power. Recently developed nonvolatile semiconductor memories are capable of storing digital information in the complete absence of power, but unfortunately, are relatively expensive. Other digital processing circuits, such as the Texas Instruments TMS 4046 and 4047 RAM (Random Access Memory) integrated circuits include a memory array which is powered separately from the decode, read, write and enable circuitry so that during a standby mode in which main power has been disconnected, a battery may be selectively coupled to only the memory portion to maintain information stored therein for a relatively long period before the battery needs to be replaced or recharged. Unfortunately, such arrangements because of their use of batteries may be undesirable for the reasons set forth above. In other digital processing circuits utilizing relatively low power consumption devices, such as C-MOS (Complementary Metal Oxide Semiconductor) devices, a capacitor may be coupled to the power supply input of the circuits to store sufficient energy to supply power to the circuits during input AC power dropouts. In U.S. Pat. No. 3,982,141 by J. A. Copeland III, there is described a digital processing arrangement including a C-MOS memory and peripheral logic elements in which, during power supply dropouts, the peripheral logic elements are selectively decoupled from the power supply line so that a capacitor coupled to the power supply line through a specially selected resistor may maintain the supply voltage for the C-MOS memory for a relatively long period. Unfortunately, where high density integrated circuits comprising relatively high power consumption devices, such as N-MOS (N channel Metal Oxide Semiconductor), P-MOS (P channel MOS), TTL (Transistor -- Transistor Logic) and I.sup.2 L (current Injection Logic) devices, are utilized to effect a cost reduction, the size of a capacitor for maintaining supply voltages for even relatively short power dropouts may be unreasonably large.
Moreover, even though it is possible to store vital information in some types of digital processing circuits during a loss of power, vital information in timekeeping systems is not static and requires updating. Therefore, the use of any one of the information storing arrangments described above without more would be undesirable.